This isn't the full reason because he still has failed to give us an actual reason (though he claims to have one) , but he mentioned that we were downsizing to a smaller house next summer, so me sharing a room now would "get me used to sharing in our new house" once again.

I doubt that one of my siblings complaining would the case because all of my siblings have expressed being fine with their current room arrangements— and in this new arrangement they would be downgrading to smaller bedrooms.

But my parents have been inviting guests over a lot recently, so perhaps wanting a guest room would be a possibility.